Odoo 11 Tree inside form group by (sale.order.line)

Question

I'm creating a module that adds chapters and subchapters to sale.order.line.

class SaleOrderLine(models.Model):
    _inherit = 'sale.order.line'

    x_capitulo = fields.Char(string="Capítulo" ,store="True")
    x_subcapitulo = fields.Char(string="Subcapítulo" ,store="True")

And for the XML

<odoo>
    <record id="sale_order_line_extended_form_view" model="ir.ui.view">
        <field name="name">sale.order.form</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="arch" type="xml">
            <xpath expr="//field[@name='order_line']/tree/field[@name='sequence']" position="after">
                <field name="x_capitulo"/>
                <field name="x_subcapitulo"/>
            </xpath>
        </field>
    </record>
</odoo>

My intention is grouping multiple sale order lines inside a sale.order object for display and accounting purpouses, it should look something like this:

Sale Order
    ....

    Order Lines
        Chapter 1
            Subchapter 1
                sale.order.line[1]
            Subchapter 2
                sale.order.line[2]
                sale.order.line[3]
            Subchapter 3
                sale.order.line[4]
                sale.order.line[5]
        Chapter 2
            Subchapter 1
                sale.order.line[7]
            Subchapter 2
                sale.order.line[6]
...

I'm trying this by passing a context to order_line object inside sale.order.form (ID sale.view_order_form)

....
<field name="order_line" context="{'group_by':['x_capitulo','x_subcapitulo']}">
<tree editable="bottom">
....
</tree>

This group_by context is not working and I can't figure out why.

I can't find a way to use group_by to a tree that is inside a form

Any help is highly appreciated.